Spinels
The term "spinel" is often employed to denote any one of a group of minerals having analogous chemical compositions which are crystallized in an isometric system with an octahedral habit. Some of the more important minerals of the spinel group are spinel, MgAl.sub.2 O.sub.4, gahnite, zinc spinel, ZnAl.sub.2 O.sub.4, franklinite (Zn,Mn.sup.2+,Fe.sup.2+) (Fe.sup.3+,Mn.sup.3+).sub.2 O.sub.4, and chromite, FeCr.sub.2 O.sub.4. These minerals also may be thought of as combinations of bivalent and trivalent oxides of magnesium, zinc, cerium lanthanum, iron, manganese, aluminum, and chromium, having the general formula: R.sup.2+ O.R.sub.2.sup.3+ O.sub.3. Thus, for example, the bivalent oxides may be MgO, ZnO, FeO, and MnO, and the trivalent oxides may be Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, Mn.sub.2 O.sub.3, La.sub.2 O.sub.3, Ce.sub.2 O.sub.3 and Cr.sub.2 O.sub.3. Spinels also can be thought of as being comprised of a first metal having a first oxidation state and a second metal having an oxidation state higher than that of the first metal and wherein each metal is appropriately associated with oxygen in a spinel lattice structure. The first and second metals may be the same metal or different metals. In other words, the same metal may exist in a given spinel in two or more different oxidation states.
Looking at spinels from yet another point of view, it also could be said that they are composed of materials having variable ratios of oxides of more than one metallic element homogeneously distributed through a crystalline matrix which is held together by loose crystalline lattice bonding. Hence, the atomic ratio of the first metal to the second metal in any given spinel need not be consistent with the classical stoichiometric formula for a given natural spinel. Hence, in an even broader sense, spinels may be thought of as being composed of bivalent and trivalent metallic oxides of continuously varying proportions nR.sup.2+ O.mR.sup.3+.sub.2 O.sub.3 wherein the ratio of n to m may vary within certain ranges. The concept of "continuously variable ratios" of atoms in a material is typified by what is commonly called a "solid solution."
It has been conjectured that the distribution of elements through a spinel crystalline structure was originally achieved in nature when eutectic compositions precipitated from molten-metal oxides and aggregated as solid masses of the eutectic compositions while, usually, leaving a remainder of a single or several surplus molten oxides. Such eutectic compounds are generally combined in the proportions of small, integral atomic ratios. One such composition of particular relevance to this patent disclosure is stoichiometric spinel, a complex compound of magnesium and aluminum oxides in equal molecular proportions as the compound Mg Al.sub.2 O.sub.4.